This disclosure relates generally to methods and apparatus for acquiring and analyzing cores from subterranean formations. More particularly, this disclosure relates to methods and apparatus for retaining a core within a coring tool.
Formation coring is a well-known process for obtaining a sample of a subterranean formation for analysis. In conventional coring operations, a specialized coring tool is used to obtain a cylindrical sample of material, or “core,” from the formation and retain that core within a core barrel so that the core can be brought to the surface. Once at the surface, the core can be analyzed to reveal formation data such as permeability, porosity, and other formation properties that provide information as to the type of formation being drilled and/or the types of fluids contained within the formation.
Conventional coring tools include a coring bit that cuts a cylindrical core from the formation. The coring bit is coupled to a barrel assembly that receives the core once it is drilled and retains the core as it is brought to the surface. Many conventional coring tools include a core catcher that operates to help retain the core within the barrel assembly once it is drilled. Many different types of core catchers have been used including wedge systems that use friction and interference fits to retain the core and valve systems that form a barrier to retain the core. One type of valve core catcher is a clamshell catcher that utilizes opposing clamshell halves that rotate inwardly to form a barrier that retains the core in a barrel assembly.
In conventional clamshell catcher systems, each opposing clamshell half has a small tab that is rotatably coupled to the assembly by a hinge pin. Once activated, the opposing clamshell halves rotate about the hinge pins until the halves contact each other to form a barrier that closes the bottom of the barrel assembly. Many conventional clamshell catcher systems include a base ring that provides a location for the mounting of the hinge pins and supports the closed clamshell halves. Due to space limitations, in conventional clamshell catcher systems, the shape of the clamshell halves is such that portions of the clamshells do not contact the base ring when closed.
Because the clamshell halves do not fully contact the base ring there can be gaps between the clamshell halves and the base ring that allow unconsolidated formation materials, such as gravel or sand, to be lost. Further, under certain loading conditions, the hinge pin and/or hinge tab can yield and allow the clamshell halves be pushed through the base ring, which can result in loss of the core.
Thus, there is a continuing need in the art for methods and apparatus for retaining cores that overcome these and other limitations of the prior art.